Holden lost to progressive candidate Matt Cartwright, an attorney who attacked the incumbent for voting against President Barack Obama's health care reform. Holden is the second House member to lose to a newcomer in the 2012 primaries. The other lawmaker was Rep. Jean Schmidt (R-Ohio). Both were also targeted by the anti-incumbent super PAC, the Campaign for Primary Accountability.

Getting a progressive Democrat into Pennsylvania's newly redrawn 17th Congressional District was a priority for progressive groups, including MoveOn and the League of Conservation Voters. The latter ran the largest independent expenditure campaign in the race, which included a $230,000 TV ad buy on broadcast and cable networks in the Scranton media market. The spot criticized Holden for opposing Obama's clean energy plan.

The League of Conservation Voters "targeted Representative Tim Holden for defeat explicitly because he has opposed policies that will curb harmful global warming pollution and build a clean energy economy," said Gene Karpinski, the group president. "Tim Holden is the first candidate this cycle to lose because he is out of the mainstream on global warming and clean energy, but he won't be the last.”

As National Journal pointed out, Cartwright was able to level the playing field in terms of money, "loaning $380,000 to his campaign and banking dozens of high-dollar donations from fellow trial lawyers and family members." Besides the League of Conservation Voters, the Campaign for Primary Accountability threw in more than $350,000 to oust Holden. The incumbent, meanwhile, received just $95,000 in outside help.

Altmire lost to Rep. Mark Critz (D-Pa.) in the state's newly redrawn 12th Congressional District. Pennsylvania's Republican-controlled legislature combined the seats of the two incumbents in redistricting, after the state lost a seat in the recent reapportionment.

Altmire was initially seen as the frontrunner in the race, but Critz received more than $83,000 in support from organized labor. Altmire also angered many progressives because he voted against Obama's health care reform; Critz did not take office until after the Affordable Care Act passed.

In a statement on Tuesday, Eddie Vale, spokesman for the AFL-CIO's super PAC Workers' Voice, said Altmire was a "huge favorite" based on the geography of the new district, but "boots on the ground and the energy of working families upset that paradigm."